Modern society continues to create exponentially increasing demands for digital information, and the communication of such information creates increasing needs for ever faster data communication speeds.
The most common form of computer-to-computer data communication in use today relies on modems and analog telephone connections. Such modems communicate data as modulated audio signals, within the voice bandwidth of the telephone network. The various elements of the telephone network treat these voice frequency signals exactly like voice type analog audio signals. This telephone-based operation provides the voice grade analog modem a unique power, the necessary connections are virtually ubiquitous. Such modems can communicate via virtually any telephone line or wireless telephone (e.g. cellular) to any other such telephone link, virtually anywhere in the world.
The telephone network was designed to provide approximately 3.3 kHz of analog voice bandwidth. The bandwidth provided is optimal for efficient transport of understandable voice information. Most switches in the telephone network now provide transport and switching in the digital form, typically using synchronous time slot interchange equipment. The digital capabilities, however, match the bandwidth optimized for voice telephone services. In particular, the switches provide 64 kb/s channel slots for calls, and the elements converting signals between the analog domain and the digital domain specifically limit the digital domain to the 64 kb/s rate.
As such, the voice telephone network simply was not designed for high-speed data communications. The small voice grade bandwidth places severe limitations on the speed of data communications. Analog telephone line modems commonly in use today operate at speeds of 28.8 kb/s or 33.6 kb/s, although newer modems are now available with 56 kb/s capabilities. Such speeds are adequate for many applications, such as E-mail and transfers of relatively small text files. However, many new multimedia applications severely tax the limits of such modems. At such speeds, large file transfers take many minutes. A user surfing the Internet often does not want to wait for a large image or video file to download at these snail-pace speeds.
Integrated Services Digital Network (ISDN) offers faster data communications and the capacity for concurrent data and voice telephone services. For ISDN service, a user obtains a digital subscriber line (DSL) termination unit connected to the customer premises end of a telephone line. The basic rate interface (BRI) DSL terminal unit provides duplex 160 kb/s digital communication with corresponding elements in the telephone network. The 160 kb/s capacity carries two bearer (B) channels, each at 64 kb/s, one data (D) channel at 16 kb/s and overhead information contained in a 16 kb/s embedded operations channel (EOC).
The telephone network switches the B-channels through the network, using the 64 kb/s synchronous time slots, in much the same way that it switches plain old telephone service (POTS) calls. B-channel data calls, however, may be switched through as end-to-end digital communications at the full 64 kb/s rate because now the digital channel rate matches the channel rate defined by the time slot interchange units within the telephone switch fabric. There are no conversions between analog and digital content within the network. The B-channels may be used separately, for example, for one voice telephone call and one data communications. Some applications also allow aggregation of the channels, to combine the B-channels to provide data communications up to the combined rate of 128 kb/s, when there is no concurrent telephone usage.
Most applications of ISDN carry the D-channel only between the customer premises and the serving central office. The D-channel typically is used for signaling, for call set-up and the like. The EOC channel goes to the central office switch, and the switch is in communication with various operations and support systems, to enable maintenance and testing type functions.
Standard ISDN equipment, such as the ISDN network termination unit at the customer premises, digital loop carrier (DLC) systems and ISDN span repeaters in the loop to the customer premises all send and receive certain codes in the EOC data stream. For example, each of these elements recognizes codes addressed to them to bridge the wires of the line and form a ‘loop-back’, effectively coupling received signals for transmission back to the central office end of the line. Subsequent addressed signals cause the element to tear down the loop-back connection. During a loop-back condition, test equipment coupled to the line can transmit signals over the line and analyze signals looped back through the line, to determine the line condition. If there are multiple elements on the line, such as repeaters and the terminal adapter, loop-back testing through the various devices may even help to identify the location of a fault.
ISDN Digital Subscriber Line (IDSL) uses the 2B1Q line coding standard for ISDN BRI circuits for data-only applications. Essentially, the two B-channels are combined and dedicated to the data service. Consequently, ISDL operates at 128 kb/s. IDSL provides this higher speed data service for line lengths up to 18,000 feet without a repeater or greater distances with ISDN repeaters, the same as standard 2B+D ISDN.
Because IDSL uses the standard ISDN line coding, customers with standard basic rate interface (BRI) type terminal adapters can use their current adapters (in a leased line mode) together with any associated equipment, for connecting to ISDL lines. At the network end of the line, the ISDL line terminates on a line card in a channel bank. However, instead of coupling the B-channels to a time slot interchange unit for switching in the same manner as ISDN, the channel bank connects the customer's B-channels over two slots on a dedicated transport to a desired data point, typically on a high-speed backbone network, such as a Frame Relay network, a super multi-mega-bit data service (SMDS) network or an asynchronous transfer mode (ATM) network. The link from the customer premises to the backbone network is a dedicated or “nailed-up” connection. The backbone network, however, provides fast access to packet-switched communication services. In this manner, IDSL lines can provide dedicated access through the particular fast packet backbone network to the Internet, private networks, or the like.
Because the two-channels provide a nailed up connection, there is no need for signaling over the D-channel, and the D-channel is not transported through the network. Also, because the connection is dedicated and does not go through a switch, there is no connection of the EOC channel on the network side. The line card and the network termination unit, the DLC terminals and/or any repeaters on the line keep the EOC channel open on the line. However, there is no network connection to enable the network to use the EOC channel. Thus in present implementations, there is no effective technique to communicate maintenance and operations messages, for example relating to loop-back testing, to the elements on the IDSL loop.
Some data communications systems utilize the in-band transmission of test commands. DDS (Digital Data Service) specifies a set of addressable command codes for transmission in the data stream over a link to customer premise data equipment. One solution of the IDSL line testing problem might be to use these or similar command codes. At least one vendor has proposed utilizing DDS codes within the data transport stream on the IDSL line, to perform certain loop-back functions. The line cards would be able to recognize these codes and perform the loop-back functions. However, this is a vendor proprietary approach, limited to the vendor's own products. The loop-back functions work only with the one vendor's equipment, i.e. that vendor's line cards. Elements not incorporating such cards, such as the network termination units or DLC systems and repeaters sold by other vendors would not recognize such codes.
To extend the in-band solution to also enable testing of repeaters, DLCs and network terminations is disadvantageous in that it would require a modification of those line components to recognize such codes. The network carrier would not be able to use standard ISDN components for the ISDL lines. Until a standard for the test codes is established, the carrier would have to limit the network connections to compatible equipment, i.e. to line components able to recognize the vendor's proprietary test codes. This would force the carrier to use only one vendor's proprietary equipment matching the vendor's channel bank and including all components from the line cards to the terminations at all of the customer premises served through that bank.
A need therefore exists for an efficient technique to communicate maintenance and testing messages to and from communication components on an IDSL line or similar line, without requiring a modification or upgrade of the components on the line or use of vendor proprietary components. The technique also should be readily adaptable to other types of digital subscriber line.